﻿<p>A spatial structure element is the generalization of all spatial elements that might be used to define a spatial structure. That spatial structure is often used to provide a project structure to organize a building project.</p>
<p>A spatial project structure might define as many levels of decomposition as necessary for the building project. Elements within the spatial project structure are:</p>
<ul>
 <li>site as <em>IfcSite</em></li>
 <li>building as <em>IfcBuilding</em></li>
 <li>storey as <em>IfcBuildingStorey</em></li>
 <li>space as <em>IfcSpace</em></li>
</ul>
<p>or aggregations or parts thereof. The composition type declares an element to be either an element itself, or an aggregation (complex) or a decomposition (part). The interpretation of these types is given at each subtype of <em>IfcSpatialStructureElement</em>.</p>
<p>The <em>IfcRelAggregates</em> is defined as an 1-to-many relationship and used to establish the relationship between exactly two levels within the spatial project structure. Finally the highest level of the spatial structure is assigned to <em>IfcProject</em> using the <em>IfcRelAggregates</em>.</p>

<p>The subtypes of <em>IfcSpatialStructureElement</em> relate to other elements and systems by establishing the following relationships:</p>
<ul>
 <li><b>Containment of elements</b> : <em>IfcRelContainedInSpatialStructure</em> by inverse attribute <em>ContainsElements</em>, used to assign any element, like
building elements, MEP elements, etc. to the spatial structure element in which they are primarily contained.</li>
 <li><b>Reference of elements</b> : <em>IfcRelReferencedInSpatialStructure</em> by inverse attribute <em>ReferencesElements</em>, used to reference any element, like building elements, MEP elements, etc. in spatial structure elements, other then the one, where it is contained.</li>
 <li><b>Reference of systems</b> : <em>IfcRelServicesBuildings</em> by inverse attribute <em>ServicedBySystems</em>, used to reference a sytem, like a building service or electrical distribution system, a zonal system, or a structural analysis system, that is assigned to this spatial structure element.</li>
</ul>
<p>The subtypes of <em>IfcSpatialStructureElement</em> relate to each other by using the <em>IfcRelAggregates</em> relationship to build the project spatial structure. Figure 2 shows the use of <em>IfcRelAggregates</em> to establish a spatial structure including site, building, building section and storey. More information is provided at the level of the subtypes.</p>

<table cellpadding="2" cellspacing="2" summary="spatial structure">
<tr valign="top"><td align="left" valign="top"><img src="../../../figures/ifcspatialstructureelement-spatialstructure.png" alt="fig1" border="0" height="701" width="551"></td></tr>
<tr><td><p class="figure">Figure 2 &mdash; Spatial structure element composition</p></td></tr>
</table>

<blockquote class="history">HISTORY&nbsp; New entity in IFC2x.</blockquote>

<p class="spec-head">Informal Propositions:</p>
<ol>
<li>The spatial project structure, established by the <em>IfcRelAggregates</em>, shall be acyclic.</li>
<li>A site should not be (directly or indirectly) associated to a building, storey or space.</li>
<li>A building should not be (directly or indirectly) associated to a storey or space.</li>
<li>A storey should not be (directly or indirectly) associated to a space.</li>
</ol>
